1. Field of the Invention
This invention relates to a radiation image storage panel and more particularly, to a radiation image storage panel comprising a support and phosphor layers provided thereon which comprise a binder and a stimulable phosphor dispersed therein.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically converting the emitted light to electric signals; and reproducing the electric signals as a visible image on a recording material such as a photosensitive film or on a displaying device such as CRT.
In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (stimulated emission) when excited with stimulating rays after having been exposed to a radiation such as X-rays. In the radiation image recording and reproducing method, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is recorded on the radiation image storage panel in the form of a radiation energy-stored image (latent image). The radiation energy-stored image can be released as stimulated emission by applying stimulating rays to the panel, for instance, by scanning the panel with stimulating rays. The stimulated emission is then photoelectrically converted to electric signals, so as to produce a visible image from the radiation energy-stored image.
It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to provide an image of high quality (high sharpness, high graininess, etc.).
In the conventional radiography the sharpness of the image depends on the spread of the emitted light (spontaneous emission) within a radiographic intensifying screen. The sharpness of the image in the radiation image recording and reproducing method utilizing a stimulable phosphor, in contrast to the conventional radiography, does not generally depend on the spread of the light (stimulated emission) emitted by the stimulable phosphor within the radiation image storage panel, but on the spread of stimulating rays therewithin. The reason can be described as follows. The radiation energy-stored image recorded on the radiation image storage panel is sequentially detected, since the stimulated emission given upon excitation with the stimulating rays for a certain period of time is detected as an output from the area of the panel to be excited therewith for said period. When the stimulating rays are spread through scattering or relfection within the panel, the stimulated emission from the area wider than the area to be excited is detected as the output therefrom.
A radiation image storage panel generally tends to provide an image of decreased sharpness, as the mixing ratio of a binder to a stimulable phosphor in a phosphor layer of the panel increases, in other words, as the amount of the stimulable phosphor contained in the phosphor layer decreases.
The radiation image storage panel is also required to have sufficient mechanical strength so as not to allow easy separation of the phosphor layer from the support, even if mechanical shocks and mechanical force in falling or bending are applied to the panel in the use. Further, since the radiation image storage panel hardly deteriorates upon exposure to a radiation and an electromagnetic wave ranging from visible light to infrared rays, the panel can be employed repeatedly for a long period. Accordingly, it is required for the panel in the repeated use not to cause such troubles as the separation of the phosphor layer from the support induced by mechanical shocks applied in the handling of radiation image storage panel in a step of exposure to a radiation, in a step of visualization of a radiation image brought about by excitation with an electromagnetic wave after the exposure to the radiation, or in a step of erasure of the radiation image information remaining in the panel.
However, the radiation image storage panel has a tendency that the bonding strength between the support and the phosphor layer of the panel decreases as the mixing ratio of the binder to the stimulable phosphor in the phosphor layer adjacent to the support decreases, in other words, as the amount of the stimulable phosphor contained therein increases.
For these reasons, it is difficult to prepare a composition for the preparation of the phosphor layer of the radiation image storage panel so as to satisfy both of the bonding strength between the support and the phosphor layer and the sharpness of the image provided thereby. In the conventional radiation image storage panel having a single phosphor layer, a radiation image storage panel capable of providing an image of high quality as well as showing a preferable bonding strength between the support and the phosphor layer is hardly obtained.